The general field of the invention is methods and means for measuring the force delivered to a load by an energy conversion machine, and more specifically, to means for indirectly measuring the torque delivered by an electric motor to a load.
A d-c electric motor is an energy conversion machine which receives electrical energy at its armature circuit and converts it to mechanical energy at its rotor by electromagnetic interaction of armature current with magnetic flux established by associated field windings. A torque is thereby induced into the rotor which causes it and the attached load to rotate. Commonly, when the motor is connected to a control system for governing rotation, rotor torque is sensed and a torque feedback signal is developed which is used by the control system to vary the amount of electrical energy supplied to the motor.
It is a fundamental principle of both servo systems and regulator control systems that the magnitude of such torque feedback signals be directly proportional to the torque delivered by the motor. Two approaches are presently used to generate a torque feedback signal for an electric motor, neither of which is entirely satisfactory. The first and most direct approach is to attach a sensing device, such as a strain gauge, directly to an element of the load. Although such a direct approach may provide an accurate indication of motor torque, the sensing apparatus commonly used are often expensive to install and maintain, particularly in applications where the sensing device is subject to severe environmental conditions.
The second, and the most common approach used to generate a torque feedback signal is to electrically sense a motor operating parameter which indirectly indicates motor torque. With d-c motors, for example, the magnitude of the motor's armature current is sensed and a torque feedback signal proportional thereto is generated. If the d-c motor has a commutating winding it is also common to sense the voltage drop across this winding for an indication of torque. Similarly, with a-c motors a torque feedback signal proportional to winding current squared can be used as an indication of the torque developed by the motor. Although prior indirect means of indicating motor torque are highly reliable and relatively inexpensive to implement and to maintain, they are not entirely satisfactory for all control purposes.
In drive systems such as that disclosed in U.S. Pat. No. 3,518,444 issued to D. E. Barber on June 30, 1970 and entitled "Control System for Excavating Equipment," the magnitude of the torque generated by a d-c hoist motor on an excavator is sensed by measuring the voltage generated across its commutating field winding. The torque feedback signal thus generated is proportional to the torque generated by the hoist motor. It has been discovered, however, that when such control systems operate as regulators to limit the torques and forces developed in the hoist drive mechanism of the excavator, the established torque limits are often exceeded during digging operations.
The excessive torques and forces which develop in excavators using prior control systems are transient in nature, and occur primarily when the dipper strikes large objects such as rocks during digging. Although they last for a relatively short period of time, such excessive transient torques and forces occur repeatedly and considerably shorten the useful life of various elements in the drive system.
Considerable effort has been made to reduce these high transient torques and forces which occur during digging and to thereby extend the useful life of the excavator front end and particularly the useful life of the hoist rope. Such past efforts have been directed primarily to means of shortening the response time of the control system. However, it has become increasingly apparent that regardless of the response time of the control system, these high transient torques and forces will continue to be generated as long as present torque sensing and measuring techniques are used. Thus, there is a need for better torque sensing and measuring techniques which will improve the response of controls to transient load conditions. Although such an improved system would find immediate application in the hoist motor drives of excavators, a control system providing such improved response would find application in a variety of drives.